Fractions for Parents Part 3

Published on June 3, 2015

Fractions for Parents Part 3

Fractions for parents part 3 by www.guruparents.com

This short video gives a refresher on subtracting fractions, so you the parent can help with your child’s homework without feeling inadequate!

1. Simple subtraction

3/7 – 2/7

When the fractions have the same base or denominator you just subtract the numerators. So in this case it’s just 3 minus 2, giving the answer 1/7.

2. Finding a common denominator

In this example, the denominator is not the same so I need to find a common one.

The lowest common denominator is 10 as 10 is divisible by both 5 and 10.

So to convert 3/5 into tenths, since I multiplied 5 by 2 to get 10 I need to do the same to the numerator. 3 x 2 = 6. 6/10.

And now I can subtract the numerators. 7 minus 6 is 1; the answer is 1/10.

3. Subtracting Mixed Numerals

Subtracting mixed numerals is often very straightforward, as is the case here.

All I need to do first is find a common denominator for the fractions. And the lowest common denominator is 6, so 4 2/3 becomes 4 4/6.

Now I just subtract the whole numbers – 4 minus 2 is 2 – and then the fractions – 4/6 minus 1/6 is 3/6. The answer is 2 3/6 which simplifies to 2 ½.

4. Subtracting harderd mixed numerals

Sometimes subtracting mixed numerals can be a bit tougher, specifically when the first fraction is smaller than the second. Here are a couple of methods to handle this.

4 1/3 – 2 3/5

First, as always get a common denominator. 15 will be the common denominator. I needed to multiply 3 by 5 to get 15, so I do the same to the numerator. 5 times 1 is 5. And over here I needed to multiply 5 by 3 to get 15, so I multiply the numerator by 3 too to get 9.

Now I have the same base so I can subtract the numerators but 5 minus 9 is going to be less than zero – which is a problem.

So, I’m going to borrow. I’ll take 1 from the whole number. So 4 becomes 3. And now I add my 1 to the fraction. Another way of writing 1 is 15/15. So 15 + 5 = 20: I now have 20/9. Incidentally a little trick here is that you can just add the numerator and the denominator together to get this: 15 + 5 = 20.

So, now I can subtract. 20 minus 9 is 11. 3 minus 2 is 1. The answer is 1 and 11/15.

Another way to approach this question is to change both fractions to improper fractions. It’s a less elegant method but it works and may be easier for some kids to do.

So what’s 4 1/3 as an improper fraction? 4 times 3 is 12 plus 1 is 13, so 13/3.

And 2 3/5 – 2 times 5 is 10 plus 3 is 13/5.

So, we need a common denominator. Again, it’ll be 15.

I multiplied 3 by 5 to get 15. So I do the same for the numerator: 13 times 5 is 65. 65/15.

And to get 5 to 15 i multiplied by 3. I do the same to the numerator: 3 times 13 is 39: 39/15.

65 minus 39 is 26. So 26/15. Finally, to convert this back to a mixed numeral. 15 goes into 26 just the once with 11 left over so we have 1 11/15 – the same as above.

So this method is just as effective, although it often involves a few harder sums.

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